Regarding pattern formation processes for diminishing the number of lithographic steps for the aforesaid semiconductor devices or image display devices represented by liquid crystal display devices (LCDs), for instance, Patent Publications 1 and 2 disclose a process with which the lithographic cycles count involved is cut down by a reflow technique, or by an ashing technique.
The aforesaid publications also give an account of a photomask having a micro-slit below the resolution limits for photolithographic light used (hereinafter called the slit mask), and a photomask having gradations with respect to photolithographic light (hereinafter referred to as the gray tone mask).
However, there is a grave problem with the fabrication of the aforesaid slit mask and conventional gray tone mask (hereinafter referred to as the prior art gray tone mask).
With the slit mask, a general light shield film capable of substantially shielding off photolithographic light, for instance, a chromium film is used and the micro-slit below the resolution limits is located at an area that is made semitransparent on a mask (for instance, see Patent Publication 3; although the mask of Patent Publication 3 is described as a gray tone mask, it is indeed a so-called slit mask). The slit in this mask, because of having a size less than the resolution limits, allows size-dependent photolithographic light to transmit through an area including a surrounding non-opening portion without forming its own image on a resist. For this reason, the slit mask functions as if there were a semitransparent film in the area with the slit formed and an area including the surroundings.
However, the slit, because of being required to be below the resolutions limits, must as a matter of course be finished to a size smaller than the main pattern of the mask, offering a grave problem that it gives an increased load on mask fabrication.
To make a wider area transparent, a lot more slits must be disposed resulting in an increase in the capacity of pattern data, which gives rise to a problem that there is an increased load on pattern formation steps and pattern defect inspection steps. This problem in turn leads to another problem such as an extended fabrication and inspection time length and an increased mask fabrication cost.
On the other hand, the prior art gray tone mask is one that uses a film capable of substantially shielding off photolithographic light plus the second film semi-transparent to photolithographic light, producing gradations (for instance, see Patent Publication 4). To fabricate this mask, an exclusive photomask blank wherein a semitransparent film and a light shield film have been stacked on a transparent substrate beforehand is used to repeat mask pattern making twice. In the first mask pattern making operation here, the light shield film and semitransparent film are etched in a stroke, and in the second mask pattern making operation, only the light shield film is etched so that the desired mask can be fabricated. Alternatively, only the light shield film may be etched in the first mask pattern making operation and, in the second mask pattern making operation, the light shield film and semitransparent film may be etched in a stroke. This prior art gray tone mask is advantageous over the slit mask in that, unlike the slit mask, there is no need of providing the micro-slit.
However, there is the need of relying upon the etching technique where, as described above, only the light shield film is removed to leave the semitransparent film behind, but there is a problem that any etching selection ratio is unavailable. For the prior art gray tone mask, there are thus some limitations to material selection for the light shield films and semitransparent films: some limited materials alone are usable, or there is no option but to provide an etching stopper layer on the semitransparent film and then provide the light shield layer on it. To make etching selection viable, there is another need of having multiple etching techniques (multiple installations, liquid chemicals, gases, etc.) at hand, which in turn offers further problems winding up with a lot more fabrication installations and steps, and mounting mask fabrication costs.
To provide a solution to the aforesaid problems, Applicant has already filed Japanese Patent Application No. 2004-195602 to come up with a gradated photomask comprising a mixture of a light shield area where a light shield film and a semitransparent film are stacked on a transparent substrate in that order, a light shield area where there is only a light shield film, a semitransparent area where there is only a semitransparent film, and a transmissive area where there is neither a light shield film nor a semitransparent film, and its fabrication process. Japanese Patent Application No. 2004-195602 says that a chromium type material generally used for photomasks is desired for the light shield layer, and a film of oxide, nitride, carbide or the like of chromium is preferable for the semitransparent film.    Patent Publication 1: U.S. Pat. No. 3,415,602    Patent Publication 2: JP(A)2002-66240    Patent Publication 3: JP(A)2002-196474    Patent Publication 4: JP(A)2002-189280
In the photolithographic step using a photomask, on the other hand, photolithographic light is reflected off the surface of the light shield film during exposure, giving rise to stray light that offers a problem that transfer precisions falls away. To get around this problem, generally available photomask blanks make much use of a double-layer arrangement having a low-reflective film stacked and formed on the surface of the light shield film. Ordinarily, a chromium film, a chromium nitride film or the like of about 50 to 150 nm in thickness is used as the light shield film, and a chromium oxide film or the like of about 20 nm in thickness is used as the low-reflective film.
As described in Japanese Patent Application No. 2004-195602, however, Applicant has now found that such problems as mentioned below arise with the fabrication of gradation photomasks using the two-layer structure mask blank having a low-reflective film on the light shield film.
The low-reflective film provided on the light shield layer has a reflectance optimized by its film quality and thickness; however, as the semitransparent film is further formed on the surface having the light shield film and low-reflective film, it gives rise to a problem that there is a reflectance change that renders the low-reflective film less effective, often resulting in an increased reflectance.
Further, when a semitransparent film pattern is formed, a reference mark previously formed in a light shield film pattern is read on a writing system at an alignment writing step to write a semitransparent film pattern in association with the read position; however, when there is the low-reflective film formed on the surface of the light shield film, there is often a problem that the reference mark formed in the light shield film is illegible on the writing system.
Still further, when a generally available photomask blank 600 of a two-layer structure, wherein, as shown typically in FIG. 8(a), a light shield film 602 of chromium nitride and a low-reflective film 603 of chromium oxide are provided on a transparent substrate 601, is used as the photomask blank, pattern etching using wet etching causes an edge portion 605 of a light shield film pattern 604 to have a reverse taper form by side etching, as shown in FIG. 8(b). Then, as shown in FIG. 8(c), as a semitransparent film 607 is formed all over the surface of the substrate while covering a low-reflective film pattern 606, it causes the semitransparent film 607 to be not deposited onto the reverse taper edge portion: the semitransparent film 607 cannot often be formed with good step coverage, often resulting in defects.